In recent years, efficiency of coal-fired power generation has been increased in order to suppress emission of carbon dioxide as greenhouse gas. To improve the power generation efficiency, it is effective to increase temperature and pressure of steam in a boiler. A material having excellent high-temperature strength and oxidation resistance is used as a material for the heat transfer tube for such a boiler. In particular, an austenitic stainless steel tube having excellent high temperature strength and oxidation resistance is used in a high temperature site.
The inside of the steel tube is exposed to high temperature steam, resulting in formation of oxidized scale mainly including iron. Hence, the inside of the steel tube is typically further improved in oxidation resistance through shot peening treatment. In light of long-term reliability over at least 10 years, the shot peening treatment is necessary to secure sufficient oxidation resistance despite causing cost increase.
On the other hand, as well known, since austenitic stainless steel is greatly thermally expanded and contracted along with temperature variation, the oxidized scales easily exfoliate due to temperature variation accompanied with start and stop of a thermal power generation plant. If the oxidized scales exfoliate, the exfoliating oxidized scales are deposited on a bent portion of the steel tube, which causes heat transfer trouble. In addition, the oxidized scales may be scattered with vapor up to a generator, leading to breakage of a turbine. Hence, it is important that oxidation resistance is improved and the oxidized scales are less likely to exfoliate, in order to meet the increased vapor temperature. It is therefore necessary to enhance resistance against exfoliation of the oxidized scales to prevent thinning (wastage). There is required a property that inhibits a phenomenon of exfoliation of the oxidized scales (referred to as “scale exfoliating resistance” in the invention) even under such a circumstance.
For example, the Patent Literature 1 discloses a technique of improving steam oxidation resistance by performing particle spray peening treatment onto steel containing a rare earth metal element (REM) added thereto. The Patent Literature 2 proposes a technique of suppressing exfoliation of oxidized scales while improving steam oxidation resistance by controlling surface roughness to be at a certain level or higher after shot peening treatment. The Patent Literature 3 proposes a technique of improving high temperature steam oxidation resistance through shot peening treatment while controlling Cr concentration in steel to be at a certain level or higher.
Among them, each of the techniques of the Patent Literatures 1 and 3 basically suppresses growth rate of scales by shot peening treatment, and therefore does not necessarily provide a sufficient effect of suppressing exfoliation of oxidized scales accompanied with start and stop of a power generation plant. The technique of the Patent Literature 2 advantageously suppresses exfoliation of the oxidized scales by controlling surface roughness of a surface subjected to shot peening. However, since repeated exfoliation of the oxidized scales gradually reduces initial roughness of a steel surface, such an effect cannot be continuously exhibited against the repeated exfoliation of the oxidized scales, and consequently sufficient properties cannot be maintained for a long period.
In this way, existing techniques of suppressing exfoliation of the oxidized scales include a technique of preventing formation of oxidized scale itself (the Patent Literatures 1 and 3) and a technique of preventing exfoliation of formed oxidized scales (the Patent Literature 2). The technique that prevents formation of oxidized scales themselves merely reduces a growth rate of scales, and does not guarantee prevention of formation of oxidized scales over a long period such as several decades. It is therefore necessary to prepare a measure for a case where the oxidized scales are formed. It is therefore necessary to provide the technique of preventing exfoliation of formed oxidized scales, but the relevant existing techniques cannot actually exhibit continuous effects on repeating exfoliation of the oxidized scales.